I rode my first coaster at age 6...a racing coaster at Euclid Beach Park. It was a very mild ride without any stomach wrenching dips.
I didn't ride a "big" coaster until I was 12. It was the BIG Dipper at Geauga Lake. I rode it to avoid intimidation. I refused to have the shortest girl in my class make me look like an idiot. She combed her hair straight-up to make herself look taller and they let her on!
Until that day, I had to see an entire ride from one vantage point, beginning to end. There was nothing more terrifying than seeing a full coaster disappear over a hill, listening to the screams and track noise, and not knowing what happen to the riders. (duh, LOL) I also wouldn't ride anything that went upside. Eventually, I rode the Rock-O-Planes.